1. Field of the Invention
The present invention concerns a controller for a radio-frequency amplifier.
2. Description of the Prior Art
A radio-frequency amplifier (RF amplifier) serves to amplify, with as little as distortion as possible a radio-frequency signal (RF signal) fed to the amplifier in order to obtain an RF signal of greater power at its output. RF signals having pulse powers of 15 to 30 kW are necessary particularly for special medical examinations by means of a magnetic resonance tomography (MRT), using an MRT apparatus. RF amplifiers are therefore employed in such apparatuses for producing RF signals having power in the aforementioned range. The RF signals are pulsed, meaning they need this power for a period ranging from a few μs to a few ms. Very precise pulse powers thus are necessary at the RF amplifier's output, especially in the case of functional MRI (magnetic resonance imaging), in order to produce high-quality medical images using the MRT apparatus. Pulse-repetition accuracies of the amplified RF signal in the order of approximately 1-4% can be achieved using a conventional transmission arrangement. The term “precise” in this context means that both the amplitude and phase of the RF signal have to meet exact specifications. To obtain accuracies of this order for the RF amplifier's RF output signal, the amplifier is provided with regulating means.
A transmission arrangement for a magnetic resonance apparatus is known from DE 103 35 144 B3 that contains control means for the amplitude and phase of the RF amplifier's RF output signal. The ratio between the RF amplifier's input and output power, which is to say of the RF signal to be amplified to the amplified RF signal (what is termed the actual amplification or actual gain), is determined by suitable detectors. The phase relationship between these two signals is also determined (termed the actual phase difference). For example an integrated gain and phase detector, for instance an AD8302 chip from the company Analog Devices, is used for that purpose. A settable attenuator and settable phase control element are used in two separate control loops for keeping the RF output signal's output amplitude and output phase constant or at the desired ratio to the RF input signal, i.e., for setting a desired amplification (desired gain) or desired phase difference.
FIG. 5 shows such an arrangement according to the prior art. A controller means 202 is connected upstream of an amplifier 200, also called an RFPA (Radio Frequency Power Amplifier). An RF input signal 206 is fed into the arrangement at an input 204. This signal proceeds through the controller means 202 and amplifier 200 via the signal line 208, to exit the arrangement as an amplified RF output signal 212 at the output 210. The controller means 202 has both a gain detector 214 and a phase detector 216.
Two measured variables, for both the RF input signal 206 and the RF output signal 212, are fed via respective signal couplers 218 and 220 assigned to the input 204 and output 210, and via corresponding measuring leads 222 and 224, both to the gain detector 214 and to the phase detector 216. The gain detector 214 determines the actual amplitude amplification 226 (actual gain) and the phase detector 216 determines the actual phase difference 228 between the RF output signal 212 and RF input signal 206. High-frequency signals (RF signals) in the range around 63 or 123 MHz, for example, are present on the measuring lines 222, 224.
The actual amplitude amplification 226 and actual phase difference 228, by contrast, as output signals of the gain detector 214 and phase detector 216, are low-frequency signals (LF signals). The actual amplitude amplification 222 and actual phase difference 228 are compared in comparators 230, 232 with a desired amplification 234 and desired phase difference 236 and appropriate correction signals are conveyed via control amplifiers 238, 240 to an attenuator 242 and a phase control element 244 in the signal line 208.
The signal paths all run separately from each other, so both the attenuator 242 and phase control element 244 are consequently provided with mutually independent, separate control loops 246, 248. An integrated gain and phase detector as mentioned above alternatively can combine the two discrete gain detector 214 and phase detector 216 components in the form of an IC 250, indicated in FIG. 5 by dashed outlining.
The RF amplifier 200, when used as an RF (radio-frequency) power amplifier in an MR system has a sharply expanding characteristic curve indicated in FIG. 5 by the characteristic curve 252. Sharply expanding means that the amplification for small input signals 206, which is to say for signals of the type having small amplitudes or, as the case may be, signal powers, is far less than for large or, as the case may be, powerful input signals 206. For different input powers Pin of the signal 206, graph (a) in FIG. 6 shows the relative amplification G of the RF amplifier 200 at a frequency of 63.6 MHz as the characteristic curve 260. Plotted above the same abscissa, graph (b) in FIG. 6 shows the amplitude error, produced by the RF amplifier 200, of the RF output signal 212 compared with the RF input signal 206 as the characteristic curve 262.
As mentioned above, MR systems operate in pulsed mode, to the characteristic curves 260, 262 shown in graphs (a) and (b) in FIG. 6 are at least partially traversed several times during each recording sequence, meaning during repeated triggering of relevant signal pulses of the RF input signal 206. In addition to variations in the amplification characteristics of the arrangement shown in FIG. 5 due to, for instance, temperature drifting etc., the regulating circuit 202 must therefore always also compensate the non-linear characteristic curves 260, 262. Due to the expanding nature of the characteristic curves 260, 262 there is also a risk that, because of the necessary large amplitude amplifications of the amplitude control element 242 at the start of a pulse, meaning in the case of low levels or powers of the RF input signal 206, the amplification may not be regulated fast enough, which will lead to overshoots and, in the worst case, safety shut-down of the amplifier 200.
It is known from the aforementioned DE 103 35 144 B3 to compensate the expanding characteristic curves 260, 262 by means of a compression network of diodes and resistors. The sharply expanding characteristic of the amplifier 200, however, can be only partially corrected by networks of this type. Such circuits, moreover, are sensitive to thermal influences and are unable to compensate phase deviations.
An analog dynamic compressor is known from DE 101 48 441 C1 by means of which far greater compression can be achieved with better temperature stability than by means of the simple diode/resistor network known from DE 103 35 144 B3. It is, however, difficult in practice to simultaneously compensate the amplitude characteristic 260 and phase characteristic 262 of the amplifier 200 using this compressor. Furthermore, both the power and circuitry requirements of an analog compressor circuit of such a type are very high for typical MR frequencies in the 63.6 or 123.2 MHz RF range. FIG. 7 is a block diagram of the cited dynamic compressor 300. The regulating circuit 202 shown in FIG. 5 has been replaced in FIG. 7 by an alternative embodiment, using an IQ control element 302 with the pre-amplifier 304. The IQ control element 302 performs the same function as the attenuator 242 and phase control element 244 shown in FIG. 5, namely harmonizing the RF input signal 206 in terms of magnitude and phase for supplying the RF in-put signal 206, modified in that way at the input 306, to the dynamic compressor 300. Pre-emphasizing by the dynamic compressor 300 for compensating the characteristic curves 260, 262 thus takes place in the radio-frequency or, as the case may be, RF path in the arrangement shown in FIG. 5.
The RF signal 206 requiring to be amplified is fed to the IQ control element and split into two partial signals having a 9020 phase offset. The partial signals then each traverse an I and a Q path. The corresponding partial signal is weighted in the I path with an I factor and in the Q path with a Q factor. The partial signals are recombined via a summing unit and the sum signal is supplied to the RF amplifier 200 (via the dynamic compressor 300). The IQ control element also influences the magnitude and phase of the RF signal 206 requiring to be fed to the RF amplifier. However, multiplying the partial signals by the I and Q factor therein in each case influences only the amplitude of the partial signals and not their phase. Because the partial signals correspond, due to the 90° phase offset, to a real and imaginary component of a complex phasor (namely their sum), the addition of the real and imaginary component, namely in the form of the IQ controller's output signal fed to the amplifier, produces a change in the respective amplitudes of the partial signals that allow total signal to be manipulated (modified) in its amplitude or phase.
Until fed into the multipliers, the I and Q factors therein operate on an LF basis in contrast to the signal paths of the partial signals in the I and Q path forming RF paths.
Because the multipliers in the IQ controller can be embodied in analog form, an IQ control element of the described type is readily able to provide the required rise times of far below 1 μs. For driving the IQ element, however, it will then be necessary to convert the intended phase and amplification changes, meaning the changes in amplitude in the RF signal (desired and actual values), into respective I and Q factors in keeping with the amplification factors for the real and imaginary component (partial signals in the partial paths). That can be done using corresponding A/D conversion and by means of digital calculations performed by a digital computer, or in an analogous way, as is described in an application of the same assignee (Siemens AG) as the present application filed simultaneously herewith and having Ser. No. 11/744,291.